Undesired heat loss from production tubing as well as uncontrolled heat transfer to outer annuli can be detrimental to the mechanical integrity of outer annuli, cause productivity losses from the well due to deposition of paraffin and asphaltene materials, accelerate the formation of gas hydrates, and destabilize the permafrost in arctic type regions.
The successful application of wellbore insulating fluids in the last several years has shown that such fluids can minimize the heat loss. Silicate foams were among the first insulating fluids. Such foams were employed in steam injection applications wherein a solution of sodium silicate was placed in a packed-off annulus, and then steam was injected down the tubing. The hot tubing caused the silicate solution to boil, leaving a coating of insulating material, silicate foam of ¼ to ½ inch thick, on the hot tubing surface. Silicate solution that remained in the annulus after steaming for several hours was removed from the annulus by displacing it with water which was removed by gas-lifting or swabbing. The foam insulator exhibited thermal conductivity of about 0.017 Btu/(hr·ft·° F.). However, difficulties were encountered in boiling off the solutions to form the foam. “Hot spots” were also observed to develop adjacent to the uninsulated couplings.
To prevent thermal refluxing, an insulating fluid that filled the entire annulus was chosen as an alternative to the gas filled annulus. Such fluids avoided unwanted heat loss as a result of reduced thermal conduction and/or convection. Oils, such as gelatinous oil based fluids, exhibited relatively low thermal conductivity (0.08 Btu/(hr·ft·° F.). For instance, the relative thermal conductivity of this type of fluid was approximately 13 percent that of water. However, environmental restrictions limited the application of such oils. Furthermore, the long-term incompatibility with various elastomers presented concerns.
As an alternative to chemical methods, vacuum insulated tubing was proposed to solve the problem of paraffin deposition in the production tubing. While insulated tubing proved to be an effective method for wellbore insulation, actual heat losses were significant. Heat loss through couplings and other internal structures such as centralizers and valves were seen to account for up to 50 percent of the total heat loss. To fully achieve the potential of insulated tubing, selected rubber-insulated couplings were tested along with a thermal pipe coating. Although improved thermal performance was obtained, maintaining the annulus dry over a long period was difficult, and, heat loss through refluxing could still occur because of damaged and scraped coating, and downhole centralizers, valves and gauges. This problem could be controlled effectively by the use of specially designed aqueous-based (oil-free) insulating fluids.
To secure the insulation of wellbore to reduce the heat transfer from the production tubing to the surrounding wellbore, internal annuli, and the riser environment, non-crosslinked insulating fluids, such as those disclosed in U.S. Pat. No. 6,489,270, proved to be non-damaging, environmentally friendly, and highly insulating. The viscosity of such fluids made it easy to blend and pump them into the annulus; the fluid density being controlled by the amount and type of dissolved salt needed to provide positive control of the wellbore pressure without the risk of solid settling and separation. Such fluids, when added either into an annulus or riser, effectively reduced undesired heat loss from the production tubing, or heat transfer to outer annuli. In some cases, heat loss from the produced fluids due to conduction and convection was reduced by more than 90% when compared with conventional packer fluids.
Other fluids, such as those disclosed in U.S. Patent Application 20040059054 were formulated to provide a viscous fluid with inherently low thermal conductivity and low convection velocity by mixing a designated amount (0.1 to 10%) of polysaccharide and superabsorbent polymers of starch grafted polyacrylates, acrylamide/acrylic acid copolymers, isobutylene/maleic anhydride, etc. into a brine mixture.
To date, such insulating fluid systems have been successful for those applications wherein the density of the brine is below 12.5 lb/gal (e.g., sodium salts). Improved insulation properties in fluids based on higher density brines (such as calcium bromide brines) have been sought. Such fluids need to be environmentally friendly, exhibit an inherently low thermal conductivity and must be capable of securing the insulation of the wellbore while reducing the amount of heat transfer from the production tubing to the surrounding wellbore, internal annuli, and riser.